Method of forming low-K interlevel dielectric layers and structures

ABSTRACT

A method of forming a structure. The method including: forming a precursor layer on a substarte, the precursor layer including a resin and, polymeric nano-particles dispersed in the resin, and a solvent, each the polymeric nano-particle comprising a multi-arm core polymer and pendent polymers attached to the milti-arm core polymer and pendent polymers attached to the multi-arm core polymer, the multi-arm core polymer immiscible with the resin and the pendent polymers miscuble with the resin; heating the precursor layer to cross-link at least about 90% of the resin thereby converting the pre-baked precursor layer to a dielectric layer; forming trenches in the dielectric layer and filling the trenches with an electrical conductor; heating the dielectric layer to thermally decompose at least acout 99.5% of the polymeric nano-particles into decomposition products and to drive the decomposition products out of the dielectric layer.

FIELD OF THE INVENTION

The present invention relates to the field of porous interleveldielectric materials used in the fabrication of integrated circuits andmethods of fabricating integrated circuits having porous interleveldielectric layers.

BACKGROUND OF THE INVENTION

As electronic devices become smaller, there is a continuing desire inthe electronics industry to increase the circuit density in electroniccomponents, e.g., integrated circuits, circuit boards, multi-chipmodules, chip test devices, and the like, without degrading electricalperformance, e.g., without introducing cross-talk capacitive couplingbetween wires while at the same time increasing speed or signalpropagation of these components. One method for accomplishing thesegoals is to reduce the dielectric constant of the dielectric material inwhich the wires are embedded. Toward this end, a new class of lowdielectric constant (low K) materials have been created. However, as thedensity of integrated circuits continues to increase, there is acontinuing need for insulating materials with ever lower dielectricconstants.

SUMMARY OF THE INVENTION

An aspect of the present invention is a method, comprising: forming aprecursor layer on a substrate, the precursor layer including a resin,polymeric nano-particles dispersed in the resin, and a solvent, each thepolymeric nano-particle comprising a multi-arm core polymer and pendentpolymers attached to the multi-arm core polymer, the multi-arm corepolymer immiscible with the resin and the pendent polymers miscible withthe resin; heating the precursor layer to a first temperature, the firsttemperature high enough to drive the solvent out of the precursor layerwithout decomposing more than about 0.5% the polymeric nano-particles,to form a pre-baked precursor layer; heating the pre-baked precursorlayer to a second temperature, the second temperature high enough tocross-link at least about 90% of the resin without decomposing more thanabout 5% of the polymeric nano-particles, thereby converting thepre-baked precursor layer to a dielectric layer, the second temperaturehigher than the first temperature; after the heating the pre-bakedprecursor layer to a second temperature, forming trenches in thedielectric layer and filling the trenches with an electrical conductor;after the filling the trenches with an electrical conductor, heating thedielectric layer to a third temperature, the third temperature highenough to thermally decompose at least about 99.5% of the polymericnano-particles into decomposition products and to drive thedecomposition products out of the dielectric layer, thereby creatingvoids in the dielectric layer and converting the dielectric layer to aporous dielectric layer, the third temperature higher than the secondtemperature; wherein the polymeric nano-particles comprise between about10% by weight and about 65% by weight of all solids in the composition;wherein the first temperature is less than about 100° C., the secondtemperature is less than about 350° C. and the third temperature is lessthan about 450° C.; wherein a thickness of the porous dielectric layerafter the heating to the third temperature is within about 2% of athickness of the dielectric layer after the heating to the secondtemperature; wherein the dielectric layer has a Young's modulus of atleast about 3.6 GPa and wherein the porous dielectric layer has apermittivity of 2.4 or less and a Young's modulus of at least about 0.8GPa; wherein the multi-arm core polymer of each the polymericnano-particle comprises a polystyrene or substituted polystyrene starmolecule having multiple polystyrene branches, each branch terminatingin either one of the pendent polymers or a hydroxyl group; wherein thependent polymers are selected from the group consisting of linearpoly(alkyleneglycol), poly(alkylene glycol methacrylate), poly(methymethacrylate), poly(acrylamide), poly(methacrylamide), poly(hydroxyethylmethacrylate) and combinations thereof; and wherein the resincomprises-(SiO₂)n-, -(R'SiO_(3/2))n-, -(R'₂SiO)n- or -(R''Si₂O₃)n-,wherein R' is selected from the group consisting of an alkyl grouphaving 1 to 3 carbon atoms, an aryl group having 6 to 12 carbon atoms,and a cycloalkyl group having 6 to 12 carbon atoms, and wherein R''isselected from the group consisting of an alkylene group having 1 to 2carbon atoms and a cycloalkylene group having 6 to 12 carbon atoms.

A third aspect of the present invention is a method comprising: forminga precursor layer on a substrate, the precursor layer including a resin,polymeric nano-particles dispersed in the resin, and a solvent, each ofthe polymeric nano-particles comprising a multi-arm core polymer andpendent polymers attached to the multi-arm core polymer, the multi-armcore polymer immiscible with the resin and the pendent polymers misciblewith the resin; heating the precursor layer to a first temperature, thefirst temperature high enough to drive the solvent out of the precursorlayer without decomposing more than about 0.5% the polymericnano-particles, to form a pre-baked precursor layer; heating thepre-baked precursor layer to a second temperature, the secondtemperature high enough to cross-link at least about 90% of the resinwithout decomposing more than about 5% of the polymeric nano-particles,thereby converting the pre-baked precursor layer to a dielectric layer,the second temperature higher than the first temperature; formingtrenches in the dielectric layer after the heating to the secondtemperature and filling the trenches with an electrical conductor; andheating the dielectric layer to a third temperature, the thirdtemperature high enough to thermally decompose at least about 99.5% ofthe polymeric nano-particles into volatile decomposition products and todrive the decomposition products out of the dielectric layer, therebycreating voids in the dielectric layer and converting the dielectriclayer to a porous dielectric layer, the third temperature higher thanthe second temperature, wherein a thickness of the porous dielectriclayer after the heating to the third temperature is within about 2% of athickness of the dielectric layer after the heating to the secondtemperature, wherein the dielectric layer has a Young's modulus of atleast about 3.6 GPa, and wherein the porous dielectric layer has apermittivity of 2.4 or less and a Young's modulus of at least about 0.8GPa.

BRIEF DESCRIPTION OF DRAWINGS

The features of the invention are set forth in the appended claims. Theinvention itself, however, will be best understood by reference to thefollowing detailed description of a illustrative embodiments when readin conjunction with the accompanying drawings, wherein:

FIGS. 1A through 1F are cross-section views illustrating fabrication ofan interlevel dielectric wiring level in an integrated circuit accordingto a first embodiment of the present invention;

FIGS. 2A through 2F are cross-section views illustrating fabrication ofan interlevel dielectric wiring level in an integrated circuit accordingto a second embodiment of the present invention;

FIG. 3 is a flowchart of the porous dielectric material formation stepsaccording to certain embodiments of the present invention;

FIG. 4 is a thermo-gravimetric analysis plot of percent weight remainingof porogen material versus temperature according to embodiments of thepresent invention; and

FIG. 5 is a plot of refractive index versus temperature of materialsused for forming a porous interlevel dielectric layer according toembodiments of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

A porogen is a material which when dispersed in a layer of anothermaterial will generate pores or voids in the layer. In one example, thepores or voids may be filled with air or another gas. The permittivityof a porous material may be modeled using Bruggeman or Looyengaapproximations. In one example, a low K dielectric layer is a layerhaving a permittivity of about 2.4 or less. The porogens according tothe embodiments of the present invention are polymeric nano-particlesand unless stated otherwise, the term porogen as used hereinafter meansa polymeric nano-particle.

A thermosetting resin is a polymer that is irreversibly cross-linked byheat. A star molecule is a molecule with a branching or multi-arm coreand has multiple polymer chain branches extending from the core.

A damascene process is one in which wire trenches or via openings areformed in a dielectric layer, an electrical conductor of sufficientthickness to fill the trenches is deposited on a top surface of thedielectric, and a chemical-mechanical-polish (CMP) process is performedto remove excess conductor and make the surface of the conductorco-planer with the surface of the dielectric layer to form damascenewires (or damascene vias). When only a trench and a wire (or a viaopening and a via) are formed the process is called single-damascene.

A dual-damascene process is one in which via openings are formed throughthe entire thickness of a dielectric layer followed by formation oftrenches part of the way through the dielectric layer in any givencross-sectional view. All via openings are intersected by integral wiretrenches above and by a wire trench below, but not all trenches needintersect a via opening. An electrical conductor of sufficient thicknessto fill the trenches and via opening is deposited on a top surface ofthe dielectric and a CMP process is performed to make the surface of theconductor in the trench co-planar with the surface of the dielectriclayer to form dual-damascene wires and dual-damascene wires havingintegral dual-damascene vias. Alternatively, trenches may be formedfirst and then vias formed in the bottom of the trenches.

FIGS. 1A through 1F are cross-section views illustrating fabrication ofan interlevel dielectric wiring level in an integrated circuit accordingto a first embodiment the present invention. In FIG. 1A, formed on a topsurface of a substrate 100 is a barrier layer 105. Formed on a topsurface of barrier layer 105 is a dielectric layer 110. Dielectric layer110 comprises a mixture of cured (cross-linked) matrix and porogenpolymeric nano-particles, hereinafter polymeric nano-particles. Thepolymeric nano-particles may advantageously have a maximum dimension ofabout 50 nm or less. The composition of and method of preparing thecomposition used to form dielectric layer 110 is described infra.

Dielectric layer 110 may be formed, for example, by (1) spin applicationof a mixture of a casting solvent, uncured resin (not cross-linked tothe extent that is it not soluble in the casting solvent) and polymericnano-particles to form a precursor dielectric layer; (2) pre-baking themixture of casting solvent, uncured resin and polymeric nano-particlesto a sufficiently high enough temperature to drive out most (i.e., about99%) if not all of the casting solvent without significantlycross-linking the resin (i.e., cross-linking less than about 5% of theresin) or significantly decomposing the polymeric nano-particles (i.e.,decomposing less than about 0.5% of the polymeric nano-particles); and(3) performing a hybridization bake at a sufficiently high temperatureto cross-link the resin (i. e., cross-link at least about 90% of theresin) without significantly decomposing any significant quantity of thepolymeric nano-particles (i.e., decomposing less than about 5% of thepolymeric nano-particles) to form dielectric layer 110.

In a first example, polymeric nano-particles comprise at least about 10%by weight of all dielectric layer 110 solids after prebake. In a secondexample, polymeric nano-particles comprise no more than about 65% byweight of all dielectric layer 110 solids after prebake.

The temperature of the hybridization bake is higher than the temperatureof the prebake. In one example the prebake temperature is between about80° C. and about 100° C. In one example, the hybridization baketemperature is equal to or less than about 200° C. In a second example,the hybridization bake temperature is equal to or less than about 350°C. In one example, at least about 90% of the resin is cross-linked afterthe hybridization bake. In one example, no more than about 5% of thepolymeric nano-particles have decomposed after the hybridization bake.

Formed on a top surface of dielectric layer 110 is a capping layer 115.Formed in substrate 100 is an exemplary contact 120. In one example,contact 120 is a stud contact to devices such as field effect andbipolar transistors, diodes, capacitors, resistors and inductors formedin substrate 100 that will be subsequently wired together to form anintegrated circuit.

Suitable materials for barrier layer 105 and capping layer 115 includebut are not limited to silicon dioxide (SiO₂), silicon nitride (Si₃N₄),silicon carbide (SiC), silicon oxy nitride (SiON), silicon oxy carbide(SiOC), carbon doped silica glass (SiCOH), plasma-enhanced siliconnitride (PSiN_(x)) or NBLok (SiC(N,H)). In one example, contact 120comprises tungsten (W). In one example, either or both of barrier layer105 and capping layer 115 are a barrier to the diffusion of copper (Cu).

In FIG. 1B, a pattern is formed in capping layer 115, exposingdielectric layer 110 wherever the capping layer was removed. Patterningof capping layer 115 may be accomplished by forming a layer ofphotoresist over the capping layer, exposing the photoresist layer toactinic radiation through a patterned photomask, developing away exposed(in the case of positive resist) regions of the photoresist layer,reactive ion etching (RIE) the capping layer, and then removing theremaining photoresist layer.

In FIG. 1C, multiple steps have been performed. First, trenches 125 and130 have been formed (for example by RIE) into dielectric layer 110using the pattern in capping layer 115 as a hardmask. Next a via 135 isformed in a bottom 140 of trench 125 exposing a top surface 145 ofcontact 120. Via 135 may be formed, for example, by a photolithographicprocess similar to that used to pattern capping layer 115 as describedsupra.

In FIG. 1D, an electrically conductive conformal layer (or liner) 150 isblanket deposited covering all exposed surfaces of trench 125, trench130, via 135, capping layer 115 and contact 120. Next, an electricallyconductive core conductor 155 is formed on top of liner 150 completelyfilling trenches 125, 130 and via 135.

In one example, liner 150 comprises a layer of Ta, TaN, Ti, TiN orcombinations thereof. In one example, core conductor 155 comprisescopper.

In FIG. 1E, a chemical-mechanical-polish (CMP) process is performed toremove all liner 150 and core conductor 155 from above a top surface 160of capping layer 115 to form a dual-damascene wire 165 and a damascenewire 170. A top surface 175 of dual-damascene wire 165, a top surface180 of damascene wire 170 and top surface 160 of capping layer 115 arerendered co-planar by the CMP process.

In FIG. 1F, a full-cure bake is performed. The full cure bake decomposesthe polymeric nano-particles forming voids 185, thus convertingdielectric layer 110 (see FIG. 1E) into a porous dielectric layer 190.The temperature of the full cure bake is higher than the temperature ofthe hybridization bake. In a first example, the full-cure baketemperature is between about 400° C. and about 435° C. Furthercross-linking of the resin may occur during the full-cure bake. In asecond example, the full-cure bake temperature is equal to or less thanabout 450° C. In one example, at least about 99.5% by weight of saidpolymeric nano-particles are decomposed by the full-cure bake. In oneexample, capping layer 115 is thin enough (after CMP) to allow diffusionof decomposition products (from the polymeric nano-particles) out ofporous dielectric layer 190 through the capping layer.

While a dual damascene process is illustrated in FIGS. 1A through 1F,the embodiments of the present invention may be applied to singledamascene processing as well. It is advantageous that any shrinkageoccurring during the aforementioned hybridization and full-cure bakes beheld to a minimum. In one example, a thickness of porous dielectriclayer 190 is within about 5% of a thickness of dielectric layer 110 (seeFIG. 1E). In one example, a thickness of porous dielectric layer 190 iswithin about 3.5% of a thickness of dielectric layer 110 (see FIG. 1E).In one example, a thickness of porous dielectric layer 190 is withinabout 2% of a thickness of dielectric layer 110 (see FIG. 1E).

FIGS. 2A through 2F are cross-section views illustrating fabrication ofan interlevel dielectric wiring level in an integrated circuit accordingto a second embodiment the present invention.

FIG. 2A is identical to FIG. 1A; however, FIG. 2B differs from FIG. 1B,in that not only has the hybridization bake been performed, but also thefull-cure bake, so dielectric layer 110 (see FIG. 2A) is converted toporous dielectric layer 190. In the second embodiment of the presentinvention, the hybridization and full-cure bakes may be combined into asingle bake. FIGS. 2C through 2F are similar to FIGS. 1C through 1Frespectively, except dielectric layer 110 (see FIGS. 1C through 1F) isreplaced with porous dielectric layer 190. FIG. 2F is identical to FIG.1F.

FIG. 3 is a flowchart of the porous dielectric material formation stepsaccording to certain embodiments of the present invention. In step 200,a precursor layer is formed on a substrate. The precursor layer includesa resin and polymeric nano-particles dispersed in the resin, whereineach polymeric nano-particle comprises a multi-arm core polymer andpendent polymers that are attached to the multi-arm core polymer asdescribed infra. The multi-arm core polymer is immiscible with the resinand the pendent shell polymers are miscible with the resin and a castingsolvent.

In step 205, the precursor layer is heated (pre-baked) to a firsttemperature. The first temperature is high enough to drive the castingsolvent out of the precursor layer without decomposing significantamounts of the polymeric nano-particles (i.e., without decomposing morethan about 0.5% of the polymeric nano-particles), thus forming apre-baked precursor layer. In one example, the first temperature isbetween about 80° C. and about 100° C., and the precursor layer is heldat the first temperature for about 1 to about 15 minutes.

In step 210, the pre-baked precursor layer is heated (hybridizationbaked) to a second temperature. The second temperature is high enough tocross-link the resin (for example, to cross-link at least about 90% ofthe resin oligmers) without significantly decomposing the polymericnano-particles (i.e., without decomposing more than about 5% of thepolymeric nano-particles), thereby converting the pre-baked precursorlayer to a dielectric layer. The second temperature is higher than thefirst temperature. In a first example, the second temperature is equalto or less than about 200° C. In a second example, the secondtemperature is equal to or less than about 300° C. In a third example,the second temperature is less than about 400° C. In one example, thepre-baked precursor layer is held at the second temperature for betweenabout 1 minute and about 60 minutes.

In step 215, the dielectric layer is heated to a third temperature. Inone example, the third temperature is high enough to thermally decomposeabout 99.5% or more of the original quantity of the polymericnano-particles in the precursor layer into decomposition products and todrive the decomposition products out of the dielectric layer, therebycreating voids in the dielectric layer and converting the dielectriclayer to a porous dielectric layer. In a second example, the thirdtemperature is high enough to thermally decompose about 99.5% or more ofthe polymeric nano-particles (remaining in the dielectric layer aftertreatment at the second temperature) into decomposition products and todrive the decomposition products out of the dielectric layer, therebycreating voids in the dielectric layer and converting the dielectriclayer to a porous dielectric layer. The third temperature is higher thanthe second temperature. In a first example, the third temperature isbetween about 400° C. and about 435° C. In a second example, the thirdtemperature is equal to or less than about 450° C. In one example thedielectric layer is held at the third temperature for between about 15minutes and about 120 minutes.

The polymeric nano-particles of the embodiments of the present inventionare advantageously comprised of a polystyrene or substituted polystyrenecore having multiple polystyrene branches, some or all of thepolystyrene branches terminating in pendent oligomers selected from thegroup consisting of linear poly(alkyleneglycol), poly(alkylene glycolmethacrylate), poly (methyl methacrylate), poly(acrylamide),poly(methacrylamide) and combinations thereof Branches not terminatingin a pendent polymer group are terminated with a hydroxyl or protectedhydroxyl group. Structure (I) illustrates an exemplary polymericnano-particle comprising a polystyrene multi-arm core polymer withpolystyrene arms terminating with linear polyethylene glycol polymers(PS-PEG). While eight arms are illustrated in structure I, there may bea larger number of arms, for example, 20 to 50 or more arms. The PEGshell is an oligomer or polymer with a degree of polymerization rangingfrom 2 to 40.

Structure (II) illustrates an exemplary polymeric nano-particlecomprising a polystyrene multi-arm core polymer with polystyrenebranches terminating with polyethylene glycol methacrylate polymers(PS-PEGM). While eight arms are illustrated in structure II, there maybe a larger number of arms, for example, 20 to 50 or more arms.

The polystyrene core (the circle in structures I and II) is illustratedin structure (III).

The cross-linked core of each molecule is small (less than about 3 wt %)of the total polymer and is composed of cross-linked structures derivedfrom divinyl benzene or other related polyfunctional olefinicallysubstituted benzene, napththalene or polynuclear aromatics. Thecross-linked central core constitutes less than about 5 mole % of thepolymer and serves mainly as a multifunctional site for growing polymerarms.

The synthesis of structure (I) is believed to be depictable as follows:

The synthesis of structure (II) is believed to be depictable as follows.

Suitable resins (after cross-linking) include organosilicate resins arerepresented by structures (IV), (V), (VI) and (VII).

wherein R′ is selected from the group consisting of an alkyl grouphaving 1 to 3 carbon atoms, an aryl group having 6 to 12 carbon atoms,and a cycloalkyl group having 6 to 12 carbon atoms, and wherein R″ isselected from the group consisting of an alkylene group having 1 to 2carbon atoms and a cycloalkylene group having 6 to 12 carbon atoms.

Resin structure (IV) may be represented by the formula —SiO₂)n—, resinstructure (V) may be represented by the formula —(R′SiO_(3/2))n—, resinstructure (VI) may be represented by the formula —(R′₂SiO)n—and resinstructure (V) may be represented by the formula —(R″Si₂O₃)n—. Theprecursor material for resin structure (IV) has the formula Si(OR)₄, theprecursor material for resin structure (V) has the formula SiR′(OR)₃,the precursor material for resin structure (VI) has the formulaSiR′R″(OR)₂ and the precursor material for resin structure (VII) has theformula (RO)₃Si—R′″—Si(OR)₃, wherein R is selected from the groupconsisting of an alkyl group having 1 to 3 carbon atoms and a cycloalkylgroup having 6 to 12 carbon atoms, and wherein R′″ is selected from thegroup consisting of an alkylene group having 1 to 3 carbon atoms and acycloalkylene group having 6 to 12 carbon atoms.

The actual resins used in compositions according to the embodiments ofpresent invention include polymers having a range of molecular weightsand may include precursor materials. After full-cure bake, furthercross-linking may take place.

The polymeric nano-particles and resin may be dissolved in a castingsolvent. The solvent may be selected from the group consisting ofpropylene glycol mono-alkyl ether alcohols, propylene glycol dialkylethers, propylene glycol mono-alkyl ether acetates, xylenes, alkanolshaving 1-6 carbon atoms, ketones having 3-8 carbon atoms, and cyclicketones having 5-6 carbon atoms. The multi-arm core polymer(polystyrene) is advantageously immiscible with the resin while thebranch terminating pendent polymers (linear polyethylene glycol,polyethylene glycol methacrylate, polymethacrylate, polyacrylamide,polymethacrylamide, poly hydroxyethyl methacrylate) is advantageouslymiscible with the resin prior to vitrification. This helps keep thepolymeric nano-particles suspended in the resin.

PREPARATION EXAMPLES Example 1 Preparation of Hydroxyl TerminatedPolystyrene Multi-arm Core Polymers

To a vigorously stirred solution of distilled styrene, anhydroustetrahydrofuran (THF) in cyclohexane, was added3-(t-butyldimethylsilyloxy)-1- propyllithium. The orange color solutionwas stirred for 30 minutes and the ‘living’ polystyrene in solution wascharacterized by withdrawing an aliquot to be quenched and precipitatedfrom methanol. To the remaining reactants, p-divinylbenzene was added togenerate polystyrene multi-arm core polymers, which contain a smallcross-linked core and multiple eminating arms of poly(styrene). The darkred solution was stirred for 30 minutes, then quenched and precipitatedinto methanol. Colorless product was collected and dissolved in THF andre-precipitated into methanol.

Example 2 Preparation of Polystyrene Core/PEG Methacrylate PolymericNano-particles

To a stirring solution of hydroxyl terminated polystyrene multi-arm corepolymers, triethylamine and dimethylaminopyridine dissolved indichloromethane, a solution of bromoisobutyryl bromide indichloromethane was added at 0° C. The mixture was stirred for 30minutes at room temperature, then refluxed for 2 hours. Product wasprecipitated into methanol multiple times to obtain pure multi-arm corepolymer initiator.

The above star initiator was combined with poly(ethylene glycol)monomethylether monomethacrylate (Molecular Weight 500) and toluene withefficient stirring in a Schlenk flask. The viscous solution was degassedrepeatedly by freeze, pump, and thaw cycles. A catalystdibromobis(triphenylphosphine)nickel (II) was added under positivenitrogen flow and the mixture was heated to 90° C. for 14 hours. Thecrude mixture was precipitated into hexane to yield a slimy greenproduct. After decanting off the solvent, the crude product wasdissolved in dichloromethane/THF mixture (50:50 by weight) and passedthrough a short column of aluminum oxide to remove the catalyst. Finalprecipitation from dichloromethane into hexane provided PEG-methacrylatefunctionalized polystyrene polymer nano-particles.

Example 3 Preparation of Polystyrene Core/linear PEG PolymericNano-particles

Anhydrous linear polyethylene glycol monomethylether was obtained byrefluxing a sample of commercial starting material in a benzene solutionwith a Dean-Stark apparatus. The solvent was removed under reducedpressure and replaced with anhydrous THF. N-butyl lithium was added dropwise at 10° C. with vigorous stirring. The solution was stirred at roomtemperature for 2 hours followed by addition of p-toluenesulfonylchloride in THF. This mixture was stirred for 16 hours at roomtemperature and then at 50° C. for 4 hours. The reaction was quenchedwith methanol and concentrated under vacuum. Crude product was passedthrough a column of neutral alumina with THF as the elution solvent.After removal of solvent, the product was re-crystallized from anethanol/isopropanol mixture and filtered with a pre-chilled frittedfunnel. The colorless solid was transferred to a round bottom flaskattached to a Kugelrohr under high vacuum at 65° C. to remove anyresidual solvent and water.

To a suspension of potassium hydride, pre-washed with pentane andsuspended in anhydrous THF, a solution of hydroxyl terminatedpolystyrene multi-arm core polymer dissolved in anhydrous THF was addedslowly. This mixture was stirred at 50° C. for 2 hours, cooled to roomtemperature, followed by the addition of PEG-tosylate in THF. Thereaction mixture was stirred at 70° C. for 16 hours. After cooling toroom temperature, a small amount of ethanol was added and the mixturewas concentrated slightly under reduced pressure. Dimethylformamide wasadded and transferred into a dialysis bag having the appropriatemolecular weight cut off for the removal of un-reacted LPEG-tosylate.The product was dialyzed against distilled water, methanol, anddichloromethane sequentially. After removal of solvent the gummy productwas dissolved in a small amount of acetonitrile and isopropanol wasadded until a tacky solid was precipitated out of the solution. Finalproduct was isolated and dried under vacuum.

Experimental Results

FIG. 4 is a thermo-gravimetric analysis plot of percent weight remainingof porogen material versus temperature according to embodiments of thepresent invention. Samples of both porogens, polystyrene multi-arn corepolymer with polystyrene branches terminating with linear polyethyleneglycol polymers (PS-PEG) and polymeric polystyrene multi-arm corepolymer with polystyrene branches terminating with polyethylene glycolmethacrylate polymers (PS-PEGM) prepared supra, were subjected tothermogravimetric analysis at a heating rate of about 10° C./minuteunder nitrogen. At about 100° C., less than 0.5% of the porogens hasdecomposed. Above about 200° C., both porogens decompose marginally andthe decomposition rate greatly accelerates above about 300° C. At 300°C. no more than about 5% of the porogens are decomposed. About 99.5% ofthe porogens are decomposed at just under about 450° C. Both porogensexhibit no residue upon heating higher than 450° C.

However, the polystyrene multi-arm core polymer with polystyrenebranches terminating with linear polyethylene glycol polymers (PS-PEG)porogen exhibits slightly steeper decomposition rate characteristicswith the onset of rapid decomposition, being shifted to highertemperatures. Aside from the difference in decomposition onset, bothporogens exhibit an extremely narrow decomposition temperature window,which is highly desirable.

FIG. 5 is a plot of refractive index versus temperature of materialsused for forming a porous interlevel dielectric layer according toembodiments of the present invention. A first porogen/resin mixture wasformulated with about 44.2 percent by volume of the polystyrenemulti-arm core polymer with polystyrene branches terminating with linearpolyethylene glycol polymers (PS-PEG) porogen and about 55.8 percent byvolume of a commercial organosilicate resin manufactured by JSRCorporation, Tokyo, Japan. This mixture was spin-coated on wafers anddifferent samples heated to the several temperatures indicated in FIG.5. As a control, a simple, linear polypropylene glycol polymer (PPG)(not a polymeric nano-particle) was formulated identical to the aboveexample in organosilicate resin and spin-coated on wafers and differentsamples heated to the several temperatures indicated in FIG. 5. Thestarting refractive index of the organosilicate/porogen mixture wasabout 1.49 at about 80° C., at which temperature most volatile solventspresent would be driven off and declined to about 1.48 at about 250° C.,at which temperature the resins cross-links (vitrifies), so the slightdecrease in refractive index between about 80° C. and about 250° C. maybe attributed to the loss of solvent and cross-linking. The startingrefractive index of the PPG polymer was about 1.45 at about 80° C. anddeclined to about 1.23 at about 250° C. (roughly the hybridizationtemperature), indicating decomposition of the PPG polymer. Therefractive index of the organosilicate resin/porogen mixture was about1.41 at about 300° C. indicating onset of decomposition of the porogenand about 1.21 at about 450° C. (roughly the full-cure temperature)indicating a porous dielectric layer had been formed.

A second porogen/resin mixture was formulated with about 37.5 percent byvolume of the polystyrene multi-arm core polymer with polystyrenebranches terminating with linear polyethylene glycol polymers (PS-PEG)porogen and about 62.5 percent by volume of the organosilicate matrixresin. This mixture was spin-coated on wafers and different samplesheated to the several hybridization/full-cure temperature combinationsindicated in Table I, and the refractive index and thickness of thelayer after heating was measured. Full-cure temperature ranged fromabout 400° C. to about 435° C.

TABLE I Thickness Thickness Refractive Refractive after afterHybridization Index after Index after Hybrid Full Cure Bake Hybrid FullCure Bake Bake Temperature Bake Bake (nm) (nm) 225° C. 1.479 1.190 807824 245° C. 1.478 1.187 803 825 265° C. 1.475 1.193 809 823 295° C.1.465 1.196 770 775 320° C. 1.472 1.201 817 824 345° C. 1.460 1.200 821823

Considering the hybridization temperatures ranges from 225° C. to 345°C. and comparing the refractive index after hybridization and full cure,a very small decrease in refractive index is observed for the afterhybrid bake samples with increasing hybridization temperature. This iscommensurate with higher degrees of condensation in the organosilicateresin, since the refractive index in the fully cured (porous) state isrelatively constant. At temperatures exceeding 300° C., the decreasinghybrid state refractive index is accompanied by a slight increase in therefractive index for the fully cured material. This would be consistentwith low degrees of porogen decomposition at the higher hybridizationtemperatures and may reflect the detrimental effect of the presence ofsmall amounts of oxygen during the hybridization bake. Oxygencontamination will decrease the stability of poly(alkylene oxides) andmay accelerate the decomposition of the poly(ethylene glycol) chainends. Nevertheless, this slight increase in the refractive index andhence the dielectric constant is negligible.

More important is the comparison of the coating thickness afterhybridization and final cure. A control polymer (PPG)/resin mixture wasformulated and compared with the second porogen/resin mixture describedsupra. The two mixtures were spin-coated on different wafers. Differentsamples were heated to the two hybridization/full-cure temperaturecombinations indicated in Table II, and the percent shrinkage in coatingthickness was measured. Full-cure temperature was about 400° C. to about435° C.

TABLE II Delta Percent Percent Percent Total Shrinkage ShrinkageShrinkage Percent Between After After Shrinkage Hybrid and 250° C. 300°C. After Full Full Cure Mixture Hybrid Bake Hybrid Bake Cure Bake BakesOrganosilicate/ 10.1 13.6 3.5 PPG Organosilicate/ 10.2 12.7 2.5 PPGOrganosilicate/ 7.2 6.5 0.7 Polymeric Porogen Nano-particleOrganosilicate/ 8.4 9.9 1.5 Polymeric Porogen Nano-particle

Shrinkage (defined in terms of a reduction in thickness) ranged fromabout 3.5% to about 0.7%. The shrinkage of the organosilicate resin/polymeric porogen nano-particle mixture was significantly less than theorganosilicate resin /PPG control mixture after hybridization bake. Theshrinkage of the organosilicate resin /polymeric porogen nano-particlemixture was significantly less than the organosilicate resin /PPGcontrol mixture after full-cure bake. The shrinkage of theorganosilicate resin /polymeric porogen nano-particle mixture betweenhybridization bake and full cure bake was significantly less than theshrinkage of the organosilicate resin /PPG control mixture. This dataindicate the embodiments of the present invention provide a relativelythermally and dimensionally stable porous dielectric material, whichshows little change in film thickness in going from the nano-hybridstate to the porous film.

Additional mechanical properties of dielectric layers formed fromporogen/resin mixtures according to embodiments of the present inventionare given in Table III. All resins were organo silsesquioxane,RSiO_(1.5), homo and copolymers silicate resins.

TABLE III Porogen/Resin Mixture 50% by Weight Porogen Young's ModulusHardness 50% by Weight Resin (GPa) (GPa) Porogen/Resin 1 After 3.6 0.19250° C. Hybridization Bake Porogen/Resin 1 After 0.8 0.11 450° C. FullCure Bake Porogen/Resin 2 After 5.2 0.26 250° C. Hybridization BakePorogen/Resin 2 After 0.8 0.11 450° C. Full Cure Bake Porogen/Resin 3After 6.4 0.42 250° C. Hybridization Bake Porogen/Resin 3 After 0.8 0.11450° C. Full Cure Bake

Because of the high Young's modulus after the hybridization bake, thecompositions of the embodiments of the present invention provide arelatively stiff material that is well suited for use in fabrication ofinterlevel dielectric layers (ILD) for damascene integrated circuitfabrication when the porosity is introduced after the metal CMP asdescribed supra. Such a process leads to minimal film shrinkage upongeneration of porosity. This feature becomes more critical as the numberof simultaneously processed layers increases.

The polymeric nano-particles as described in the various embodiments ofthe present invention may be advantageously be radical block copolymerswith less than about 5 mole % of bi-functional cross-linking reagentswith very low cross-linking, levels with the cross-linking localizednear the center of the particle. This structure allows efficient andclean (with little char residue) decomposition of the polymericnano-particles, high decomposition temperatures and foaming efficienciesin excess of 90%. The low level of and localized cross-linking and theblock copolymer nature of the polymeric nano-particles of theembodiments of the present invention distinguishes these materials frommicro-emulsion particles prepared by top-down procedures. The inner coreof the polymeric nano-particle, being comprised of styrene or alkylsubstituted styrenes, is highly incompatible with the matrix(organosilicate) resin and delivers high thermal stability. The highthermal stability allows the matrix to be cured to higher temperatures,resulting in higher vitrification levels. This leads to very lowshrinkage when converting the cured matrix to a porous dielectric.Additionally, the low cross-link density of the nanoparticles minimizeschar upon decomposition, yielding optimum permittivity values.

The compositions and the embodiments of the present invention are usefulfor both (1) fabrication of interlevel dielectric layers (ILD) fordamascene integrated circuit fabrication where trenches and vias areetched in the non-porous (hybrid) dielectric material, metal isdeposited and planarized, and the porosity then generated, as well asfor (2) fabrication of ILD for damascene integrated circuit fabricationwhen the porosity is introduced before trench formation, providingimproved dimensional control and lower film stresses because of reducedshrinkage. In both applications, the high thermal stability of thenano-particles porogens is advantageous because it increases poreforming efficiency (foaming efficiency).

In the porosity after metallization process, several wiring levels maybe fabricated with hybridization baked only dielectric layers andporosity may be generated in the dielectric layers of the several wiringlevels by full cure baking the several wiring levels simultaneously.

Thus, the embodiments of the present invention provide an insulatingmaterial with high strength during mechanical processing, very low filmshrinkage during pore formation, and a low dielectric constant afterpore formation, and is thus easily integrated into conventionalintegrated circuit fabrication processes.

The description of the embodiments of the present invention is givenabove for the understanding of the present invention. It will beunderstood that the invention is not limited to the particularembodiments described herein, but is capable of various modifications,rearrangements and substitutions as will now become apparent to thoseskilled in the art without departing from the scope of the invention.Therefore, it is intended that the following claims cover all suchmodifications and changes as fall within the true spirit and scope ofthe invention.

1. A method, comprising: forming a precursor layer on a substrate, saidprecursor layer including a resin, polymeric nano-particles dispersed insaid resin, and a solvent, each said polymeric nano-particle comprisinga multi-arm core polymer and pendent polymers attached to said multi-armcore polymer, said multi-arm core polymer immiscible with said resin andsaid pendent polymers miscible with said resin; heating said precursorlayer to a first temperature, said first temperature high enough todrive said solvent out of said precursor layer without decomposing morethan about 0.5% said polymeric nano-particles, to form a pre-bakedprecursor layer; heating said pre-baked precursor layer to a secondtemperature, said second temperature high enough to cross-link at leastabout 90% of said resin without decomposing more than about 5% of saidpolymeric nano-particles, thereby converting said pre-baked precursorlayer to a dielectric layer, said second temperature higher than saidfirst temperature; after said heating said pre-baked precursor layer toa second temperature, forming trenches in said dielectric layer andfilling said trenches with an electrical conductor; after said fillingsaid trenches with an electrical conductor, heating said dielectriclayer to a third temperature, said third temperature high enough tothermally decompose at least about 99.5% of said polymericnano-particles into decomposition products and to drive saiddecomposition products out of said dielectric layer, thereby creatingvoids in said dielectric layer and converting said dielectric layer to aporous dielectric layer, said third temperature higher than said secondtemperature; wherein said polymeric nano-particles comprise betweenabout 10% by weight and about 65% by weight of all solids in saidcomposition; wherein said first temperature is less than about 100° C.,said second temperature is less than about 350° C. and said thirdtemperature is less than about 450° C.; wherein a thickness of saidporous dielectric layer after said heating to said third temperature iswithin about 2% of a thickness of said dielectric layer after saidheating to said second temperature; wherein said dielectric layer has aYoung's modulus of at least about 3.6 GPa and wherein said porousdielectric layer has a permittivity of 2.4 or less and a Young's modulusof at least about 0.8 GPa; wherein said multi-arm core polymer of eachsaid polymeric nano-particle comprises a polystyrene or substitutedpolystyrene star molecule having multiple polystyrene branches, eachbranch terminating in either one of said pendent polymers or a hydroxylgroup; wherein said pendent polymers are selected from the groupconsisting of linear poly(alkyleneglycol), poly(alkylene glycolmethacrylate), poly (methyl methacrylate), poly(acrylamide),poly(methacrylamide), poly(hydroxyethyl methacrylate) and combinationsthereof; and wherein said resin comprises —(SiO₂)n-, —(R′SiO_(3/2))n-,—(R′₂SiO)n- or —(R″Si₂O₃)n-, wherein R′ is selected from the groupconsisting of an alkyl group having 1 to 3 carbon atoms, an aryl grouphaving 6 to 12 carbon atoms, and a cycloalkyl group having 6 to 12carbon atoms, and wherein R″ is selected from the group consisting of analkylene group having 1 to 2 carbon atoms and a cycloalkylene grouphaving 6 to 12 carbon atoms.